1. Field of the Invention
The present invention relates to a light-emitting element, a light-emitting device, a display device, a lighting device, and an electronic device.
2. Description of the Related Art
As next generation lighting devices or display devices, display devices using light-emitting elements (organic EL elements) in which organic compounds are used as light-emitting substances have been developed because of their advantages of thinness, lightweightness, high speed response to input signals, low power consumption, and the like.
In an organic EL element, voltage application between electrodes, between which a light-emitting layer is provided, causes recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Since the wavelength of light emitted from a light-emitting substance depends on the light-emitting substance, use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements which exhibit various wavelengths, i.e., various colors.
In the case of display devices which are used to display images, such as displays, at least three-color light, i.e., red light, green light, and blue light is necessary for reproduction of full-color images. Furthermore, in application to lighting devices, it is ideal to obtain light thoroughly covering the visible light region for obtaining a high color rendering property, and light obtained by mixing two or more kinds of light having different wavelengths is used for lighting application in many practical applications. It is known that, with a mixture of three-color light, i.e., red light, green light, and blue light, white light having a high color rendering property can be obtained.
The wavelength of light emitted from a light-emitting substance depends on the substance, as described above. However, important performances as a light-emitting element, such as a lifetime, power consumption, and emission efficiency, are not only dependent on a light-emitting substance but also greatly dependent on the materials in the layers other than a light-emitting layer, an element structure, and the like.
As is generally known, the generation ratio of a singlet excited state to a triplet excited state in a light-emitting element using electroluminescence is 1:3. Therefore, a light-emitting element in which a phosphorescent material capable of converting the triplet excited state to light emission is used as a light-emitting substance can theoretically obtain higher emission efficiency than a light-emitting element in which a fluorescent material capable of converting the singlet excited state to light emission is used as a light-emitting substance.
As a host material in a host-guest type light-emitting layer or a substance contained in each transport layer in contact with a light-emitting layer, a substance having a higher singlet excited state or a higher triplet excited state than a light-emitting substance is preferably used for efficient conversion of excitation energy to light emission from the light-emitting substance.
However, most substances that are used as a host material of the light-emitting element are fluorescent materials, in which transition between the states with different spin multiplicities is forbidden. The triplet excited state is normally at a lower energy level than the singlet excited state having the same excited level, which means that a host material for obtaining phosphorescence needs to have a higher singlet excited state than a host material for obtaining fluorescence of the same wavelength.
Therefore, a host material and a carrier-transport material each having a further wider band gap are necessary in order to efficiently obtain phosphorescence of a shorter wavelength. A light-emitting element that includes such a material and emits phosphorescence of a short wavelength (e.g., blue phosphorescence) has a high deterioration rate and thus experiences difficulty in actual use. Against this backdrop, lifetime extension of a phosphorescent light-emitting element by devising an element structure has been under study (for example, see Patent Document 1).